Active matrix organic light emitting diode (AMOLED) display has some advantages such as compact size, high efficiency and high color saturation, so that AMOLED has become one of the mainstreams in the display technology. However, because the manufacturing variation or the threshold voltage variation resulted from the aging degradation of transistors, the driving current as well as the crossing voltage of the light emitting diode may be unstable and the consequentially the associated AMOLED display panel may have brightness non-uniformity issue. In addition, the AMOLED is driven by current, poor display quality may also happen when the gate-source crossing voltage of transistor is affected by the increase of the internal resistance of the AMOLED.